Active matrix substrates are conventionally known that include (i) X-Y matrix electrode wires constituted by scanning electrodes and signal electrodes arranged to form a lattice and (ii) switching elements, such as TFTs (thin film transistors), provided at the crossings where electrode wires cross, so as to scan drive the switching elements sequentially, one scanning electrode at a time. Such active matrix substrates are used in various applications including flat panel displays and flat panel sensors to take advantage of their flatness, high driving capability, and other superior features.
A specific example is shown in FIGS. 6(a) and 6(b) depicting a liquid crystal display of an active matrix type made up of active matrix substrates together with liquid crystal which is an electro-optical medium. The display has found a wide variety of applications in many fields such as office automation and audio visual equipment. FIG. 7 shows another example, depicting an X-ray sensor of a flat panel type, currently under development, made of the active matrix substrate together with a photoconductor for converting X-ray energy into electric charges. The X-ray sensor readily provides X-ray image data in digital form and delivers a capability to take moving pictures. Researches are thus eagerly conducted to develop X-ray sensors which could replace X-ray films.
However, in the flat panel display, the flat panel sensor, etc., an attempt to increase the display area or precision would require higher drive frequencies and cause a greater resistance and parasitic capacitance to develop in electrode wires (bus lines) on an active matrix substrate. For example, in the flat panel display, a greater resistance and parasitic capacitance in the electrode wire undesirably delays a drive signal and introduces non-uniformity in the displayed image. In the flat panel sensor, a greater resistance and parasitic capacitance in the electrode wire undesirably leads to deterioration in the S/N ratio of a reading signal which is picked up through a signal electrode to read a weak signal produced by the photoconductor.
To solve these problems by way of reducing the resistance in the electrode wire, the electrode wire is made of relatively low resistance aluminum and with an increased thickness.
However, typically, the electrode wire on the active matrix substrate is formed by first depositing a metal thin film on the substrate using a vacuum film forming device that can perform vacuum vapor deposition, such as sputter vapor deposition, and then etching out a predetermined wiring geometry (pattern). Therefore, an increase in the thickness of the electrode wire by way of the foregoing techniques entail the following problems.
(1) A typical vacuum film forming device can handle a single wafer at a time and must sacrifice its throughput (performance) to form thicker electrode wires, which brings down the productivity in producing active matrix substrates. (2) Since a metal thin film must be deposited on the entire surface of the substrate before removing unnecessary parts by etching, etching a relatively thick metal film is a time consuming process and produces large amounts of removed waste metal (material).
For these reasons, in view of productivity, cost, and other contributing factors, the conventional techniques are both difficult and impractical to provide electrode wires with over a 500 nm thickness (to increase the thickness of the deposited film). In other words, the conventional techniques have problems such that it is very difficult in practice to increase the thickness of the electrode wire on the active matrix substrate for reduction in the resistance of the electrode wire and eventually to increase the display area and precision of the flat panel display and flat panel sensor.